Fish age and size distributions and species composition in a large,hydropeaking Prairie River

Fluctuations in river flows result from diverse natural and/or anthropogenic causes. Hydropeaking, an important anthropogenic flow alteration, results from the rapid increase or decrease of water releases from reservoirs at hydroelectric power stations to meet variable demand for electrical power, thereby altering the flow regime of the river downstream of the hydroelectric power station. Hydropeaking causes short‐term, artificial fluctuations in flow on an hourly, daily, and/or weekly basis. The frequent and regular occurrences of these high and low flow events are fundamentally different from natural flood and drought events and may affect fish fauna. We compared the fish species composition and fish age and size distributions in the Saskatchewan River (Saskatchewan, Canada) downstream of a hydropeaking facility with results from an unaffected Reference Site situated upstream of the reservoir. Lower fish diversity was observed in the 2 downstream sites (Sites 1 and 2, number of species = 11 and 9, respectively) closest to Generating Station in comparison to Site 3 (n = 15) situated further downstream and the upstream reference site (n = 13). There was no difference in the age–length relationship of any of the fish species above and below the Generating Station suggesting that fish grew at the same rates. However, lower numbers of small‐bodied and juvenile fish were caught downstream of the Generating Station suggesting the possibility of increased mortality, decreased habitat suitability, or altered behaviour of small fish downstream of the dam. These data illustrate potential impacts of hydropeaking power stations and has management implications.